This invention comprises a method of automatically making a complete analysis of the polarization of light, and digitizing the four parameters which characterize it, all in real time, and a compact stand alone device to do this. This invention also calculates the parameters in several different representations, and displays them. The display could be numerical or pictorial at the choice of the operator.
Although the human eye is not sensitive to the polarization of light, polarization phenomena are ubiquitous in nature. For example, light reflected from many surfaces is partially polarized, and so is the blue sky. In the past, polarizing filters have been employed to exploit these phenomena, to the advantage of users. For example, for night driving, polarized spectacles have been used to eliminate the glare from headlights reflected from a wet pavement, and photographers have used a polarizing filter to reduce or eliminate reflections from windows. These are, however, relatively crude applications, limited in scope.
The use of a (generalized) polarization filter, e.g. in a surveillance camera, could improve contrast. The difficulty here is to know beforehand what filter to use. If it happens that the choice was not the best, there is no way of changing it after the event. Other disadvantages of the use of a single filter in this situation are: that it might be desirable to use different filters for different parts of the scene, and that no physical filter can cut out the unpolarized component of the light, since photons (light) obey the quantum mechanical rules of interaction with physical objects. [Paul Adrien Maurice Dirac. xe2x80x9cPrinciples of Quantum Mechanics,xe2x80x9d 3rd. Ed. Pp. 4-7 Oxford University Press 1947]
An improvement on the traditional use of xe2x80x9cfixedxe2x80x9d filters can be made using liquid crystal waveplates, whose optical properties can be changed electrically, but the same two disadvantagesxe2x80x94not knowing in advance, and the inability to eliminate the unpolarized componentxe2x80x94still remain.
This invention describes a method to get around these difficulties by not throwing away any of the information concerning the polarization.
When it is desired to analyse the polarization of a light beam, traditionally four measurements are made of the intensity of light passed by four different xe2x80x9cfilters.xe2x80x9d This is traditionally done by placing a quarter wave plate and a polarizing filter in the beam in four different orientations [J. M. Stone. xe2x80x9cRadiation and Optics.xe2x80x9d p.540 McGraw Hill, 1954]. A prescription for finding these parameters (e.g. Stokes"" original prescription) can be followed automatically by rotating the various elements e.g. by a servo motor; the intensities are then measured in sequence. In order to avoid moving parts, the beam can be split and the measurements made at the four different exits.
The disadvantages of beam splitting are, first that it is bulky, and secondly there is a problem concerning the relative intensities of the split beams, and the relative sensitivities of the detectors. The disadvantages of moving elements are, firstly that mechanical vibration might upset the optical alignment, and secondly, if the beam goes through a different part of an optical element after it has been moved, the transmission might be different due to dirt and imperfections. If an element has to be inserted in the beam, as in Stokes"" original prescription, there is a loss of light due to reflection at the surfaces of that element.
All automatic instruments offered to date to analyse the polarization of light require an external computer (e.g. a desktop or a laptop as a minimum) to do the necessary calculation of the polarization parameters. This is an expensive addition and makes the system not very portable. This invention will contain its own computer, a microprocessor, enabling it to be a truly portable stand alone instrument.
In order to determine the complete state of polarization of light, four polarization states, against which the incident light is compared, are chosen in a manner to provide the maximum accuracy in the final parameters. They are also chosen so that a device can be constructed to access these four polarization states with maximum efficiency.
The light first passes through a xcex/2 waveplate, next a xcex/3 waveplate, and next a polarizing filter before falling on to a light sensitive element. The waveplates are liquid crystal waveplates which can be switched electrically between a birefringent and an isotropic state.
The device includes a control module, containing electronic circuits to actuate the waveplates, to process the output of the light sensitive element, a display screen to output the results, and a microprocessor to coordinate the operation of these elements.
The waveplates, polarizing filter and the light sensitive element comprise the measuring head; interchangeable measuring heads, designed for special purposes, can be connected to the control module.
With a photodiode as the light sensitive element, the instrument is a polarimeter; the parameters of the polarization can be displayed in whatever representation the operator selects. This is a portable stand alone polarimeter.
With a digital camera for the light sensitive element, the instrument produces four scenes which can be processed digitally to produce a scene as seen through any arbitrary polarizing filter. Such a filter is not limited to filters which can be realized physically. In particular, the unpolarized component of the light can be filtered out; this is not possible with physically realizable filter elements.
Objects and Advantages
This invention comprises a method of automatically making a complete analysis of the polarization of light, and digitizing the four parameters which characterize it, all in real time, and a compact stand alone device to do this. For use as e.g. a polarimeter, an ellipsometer or a saccharimeter, this stand alone device will have a screen to display the results of such a measurement in any one of several representations at the choice of the operator. It will have a port for connection to a computer so that, instead of being used only in the manual stand alone mode, it can be automatically operated by the computer as part of a larger system.
Embodiments are not limited to measuring the polarization of a single beam of light. Simultaneous measurements can be made on several spatially separated beams. For example, another application of this method of analysis of polarization is as a filter element to be placed in front of the lens of a digital camera, and each pixel is to be regarded as an independent light sensitive element. These four parameters of each pixel could then be stored and modified in a manner equivalent to the effect of a physical filter placed before the lens, and a scene synthesized from these modified parameters. This is a digital filter, and its advantages are that several different modifications can be made to the parameters, equivalent to different physical filters placed in front of the camera lens, and the choice of the best filter can be made after the picture is taken.
These modifications are not limited to mimicking a physical filter, since physical filters of light are restricted by the laws of measurement of quantum mechanics [Paul Adrien Maurice Dirac. xe2x80x9cPrinciples of Quantum Mechanics,xe2x80x9d 3rd. Ed. Pp. 4-7 Oxford University Press 1947]. In particular, it is possible with this system to eliminate all, or part, of the unpolarized component of the light. No physical filter can eliminate the unpolarized component.
Other advantages are that it has no moving parts, and different interchangeable arrangements of the optical parts can be constructed for special purposes.
With the possibility of downloading special programs into the microcontroller, it is a truly versatile instrument.